Bone fractures are often repaired by securing a bone plate across the fracture. Depending upon which bone is to be treated, the bone plate may be straight or curved to match the contour of the bone for which it is designed. Bone plates may also be provided in many shapes and sizes. In cases where a bone is severely comminuted or if bone segments are missing, the use of bone plate and screw systems promotes healing of the fracture by providing a rigid fixation or support structure for the bone.
Bone plates may be secured to the bone in a number of ways. An existing solution is a plate and screw system where screws having threaded heads, called locking screws, are locked in the plate. The locking screw is threaded through an opening in the plate and into the bone. The locking screw is then secured to the bone plate via threads in the screw head that cooperate with threaded openings in the bone plate. Because the threads on the head of the locking screw interdigitate with threads in the plate, the plate and screws(s) form one stable system and secure the plate with respect to the bone in rigid fixation. Locking screws can achieve angular and axial stability and eliminate the possibility for the screws to toggle, slide, or become dislodged in situ, thereby reducing the risk of postoperative loss of fracture reduction.
Although locking screws may reduce the incidence of loosening, the threads on the head of a locking screw properly engage the threads of an opening only when the locking screw is inserted in a single angular orientation pre-determined by the axis of the threaded opening. Given this uniaxial relationship, locking screws have limited versatility.
One such example of limited use occurs when treating a comminuted fracture where multiple bone fragments are in irregular positions or otherwise displaced. Although a surgeon may wish to obtain the benefits of a locking screw, the pre-determined angle at which the locking screw extends from the plate may not be the angle that would allow the surgeon to “grab” (or seize, or otherwise secure) a desired bone fragment. In this case, the surgeon may need to secure the plate to the bone somewhere else or use a non-locking screw.
A non-locking screw has a head that is not threaded, but is instead round and smooth. Non-locking screws can be used in either threaded openings or non-threaded openings. Because there are no threads on the head, a non-locking screw is not threaded with or secured to the plate. Thus, one advantage of non-locking screws is that they can be inserted at various angles because they are not limited by the threaded engagement of locking screws with the bone plate. Non-locking screws present some disadvantages, however. For example, a non-locking screw is not optimal if the surgeon desires the rigid stable construct of a locking screw and plate. Non-locking screws can loosen, causing the screw to toggle, slide, or become dislodged.
There are bone plating systems that provide the surgeon with the option of choosing a non-locking or a locking screw. Some systems provide plates with both threaded holes (that may receive either locking screws or non-locking screws) and non-threaded holes (for non-locking screws). There are also systems that provide partially threaded slots to allow either non-locking or locking screws to be used interchangeably in the same slot. Such combination slots provide surgeons with the intra-operative choice about whether to use the plate with locking screws, non-locking screws, or with a combination of both. These combination slots typically have a partially threaded opening that can receive either a compression screw or a locking screw. Because these combination slots are only partially threaded, however, the locking screw(s) may not be able to maintain the fixed angular relationship between the screw(s) and plate under physiological loads. Specifically, the locking screws within the plate are only partially captured and thus only partially surrounded by threads. Under high stress and loading conditions, the slot may distort and allow the fixed angular relationship between the locking screw and plate to change. This can result in loss of fixation or loss of established intra-operative plate orientation. Moreover, the locking screw can still only be inserted at a single angle—the predetermined angle defined by the manufacturer.
Additionally, current bone plate and screw systems still limit a surgeon's ability to both lock a fastener with respect to the bone plate, but still allow the fastener to extend from the bone plate at various angles. Locking screws lock into the plate, but only in a single angular configuration, and non-locking screws allow various angle configurations, but they do not provide a stable construct with the plate. Accordingly, none of these options allow a surgeon to capture bone fragments that do not align with the axis of the opening provided on the plate in a rigid fashion. Thus, currently available options can still lead to misalignment and poor clinical results.
There have been some attempts to provide polyaxial locking systems. One effort includes providing holes that accept fixed angle locking pegs and multidirectional locking pegs, with a threaded cap inserted over the multidirectional peg to hold it in the desired angular orientation within the hole. Such a system can be cumbersome to use because, although the multidirectional peg can be inserted at any angle, the surgeon then needs to thread a small cap onto the top of the peg head and into the plate, requiring an extra step, extra time, and extra instrumentation. Such systems also fail to allow the use of non-locking members in conjunction with the locking and multidirectional pegs.
Other systems that have attempted to offer polyaxial fixation include providing a bone plate with inserts at the hole peripheries made out of a deformable material, with the remaining part of the plate made of titanium. The plate is manufactured and the inserts are then pushed into the hole peripheries. When screws are inserted, the inserts are compressed between the screw heads and the edges of the plate's holes, thereby holding the screws and inserts in place. Challenges with such systems are that they cannot be used with non-locking screws and plates with deformable inserts are more expensive to manufacture than regular bone plates. Accordingly, there exists a need for an improved bone plating system that overcomes the deficiencies of the prior art. In particular, there exists a need to provide a plating system that allows the surgeon to choose the angle at which a screw or fastener is inserted through, and rigidly affixed in, an opening of a bone plate.